With the end of cheap oil and the mounting peak of world oil production, it is recognized that petroleum is a non-renewable resource and will eventually be depleted. This realization has sparked a renewed interest in the development of renewable sources for fuel. This is particularly true in the case of aviation fuels.
In the United States, the Federal Aviation Administration (FAA) is responsible for setting the technical standards for aviation fuels through (ASTM) International. Any new fuel must comply with an existing fuel specification. For example, the FAA uses as a standard for aviation gasoline ASTM D910-Grade 100LL. This is true whether the new fuel is based on petroleum or a chemical or chemical combination.
Ethanol-based fuels for internal combustion engines have been available for roughly five decades. The State of California originated mandatory oxygenation of motor fuels, which includes ethanol-based fuels, partly to decrease the wholesale cost of fuel, and to a lesser extent to reduce air pollution per gallon of gasoline consumed. Effectively, since ethanol-based fuels have lower energy, pollution is generally increased per mile. A key benefit of ethanol-based fuels is that they have a slightly higher octane number than ethanol-free gasoline. This is the reason many oil companies provide high ethanol containing premium fuels and lower ethanol regular grades of gasoline. Renewable fuels made from some chemical species other than ethanol have been found to exhibit significantly higher octane numbers and increased energy per unit volume when compared to commercial fuels and ethanol-based fuels.
Octane (Power)
Octane number is a measure of the effectiveness of power production. It is a kinetic parameter, therefore difficult to predict. The American Society for Testing and Materials compiled volumes of experimental octane data (for pure hydrocarbons) for the Department of Defense in the 1950's. The method used to obtain this dynamic parameter is discussed in the next paragraph. 2,2,4-trimethyl pentane (isooctane) has a defined octane number of 100, and n-heptane has a defined octane number of 0, based on experimental tests. Octane numbers are linearly interpolated by this method; hence predictions for mixes can be made once pure sample values are determined.
Automobile gasoline is placarded at the pump as the average of Research and Motor octane numbers. These correlate to running a laboratory test engine (CFR) under less severe and more severe conditions, respectively. Effective octane numbers lie between the Research and Motor octane values. Aviation gasoline has a “hard” requirement of 100 MON (motor octane number); ethanol has a MON of 96, which makes its use only viable when mixed with other higher octane components. Conventional 100LL (i.e., 100 octane low lead) contains a maximum of 3 ml of tetraethyl lead per gallon to achieve the desired octane rating.
Range (Energy)
The inherent energy contained within gasoline is directly related to mileage, not to octane number. Automobile gasoline has no energy specification, hence no mileage specification. In contrast, aviation fuels, a common example being 100 LL (100 octane low lead), have an energy content specification. This translates to aircraft range and to specific fuel consumption. In the octane examples above, i-octane and n-heptane had values of 100 and 0, respectively. From an energy perspective, they contain heat of combustion values of 7.84 and 7.86 kcal/ml, respectively, which is the reverse of what one would expect based on power developed. Aircraft cannot compromise range due to the sensitivity of their missions. For this reason, energy content is equally important as MON values.
The current production volume of 100LL is approximately 850,000 gallons per day. 100LL has been designated by the Environmental Protection Agency (EPA) as the last fuel in the United States to contain tetraethyl lead. This exemption will likely come to an end in the near future.
Although discrete chemical compounds have been found to satisfy the motor octane number for 100LL octane aviation gasoline, they fail to meet a number of other technical requirements for aviation gasoline. This is true, for example, for isopentane, 90MON, and trimethyl benzene 136MON. For example, pure isopentane fails to qualify as an aviation fuel because it does not pass the ASTM specification D909 for supercharge ON, ASTM specification D2700 for motor octane number, and ASTM specification D5191 for vapor pressure. Pure sym-trimethylbenzene (mesitylene) also fails to qualify as an aviation fuel because it does not pass ASTM specification D2386 for freeze point, ASTM specification D5191 for vapor pressure, and ASTM specification D86 for the 10% distillation point.
It is of paramount importance that industry continues to progressively improve its environmental performance and lessen impacts to the global ecosystem, while continuing to reduce operating costs. Aviation recognizes these challenges must be addressed to ensure industry viability and is actively seeking to provide technologically driven solutions. Bio-derived jet fuel is a key element in the industry strategy to address these challenges.
Significant progress has been made in verifying the performance of Synthetic Paraffinic Kerosene (SPK) made from sustainable sources of bio-derived oils, after catalytic cracking and hydrogenation, that can be used in commercial aircraft at a blend ratio of up to 50 percent with traditional jet fuel (Jet A or JP-8).
Current alternative jet fuel certification targets are paraffinic alternative fuels used in 50/50 blends with conventional jet fuels, but the availability of synthetic aromatics (like mesitylene) enables the adjustment of the properties of paraffinic fuels, plus enables the potential of fully renewable fuels.
In addition, there is a significant amount of ongoing alternative aviation fuel research, both civilian and military, aimed at developing “drop-in” replacements for current petroleum-derived fuels. “Drop-in” means a fuel that is functionally equivalent to current fuels, requiring no aircraft hardware or handling changes.
Initial targets for certification of such fuels are Synthetic Paraffinic Kerosene (SPK) and Hydroprocessed Renewable Jet Fuel (HRJ), both as 50/50 with conventional petroleum-derived jet fuels. SPK and HRJ contain fully saturated linear alkanes in the C12-C22 range. These two processes typically produce a hydrocarbon jet fuel predominantly consisting of n-paraffins and iso-paraffins. Commercially, alternative fuels are added to ASTM D7566 when certified. These paraffinic fuels are not “drop-in” jet fuel for a number of reasons: first, their density falls below allowable 0.775-0.84 range; and second, they tend to cause fuel leaks through o-ring seals (due to the lack of aromatic components).
Currently, these shortcomings are avoided by blending the paraffinic fuels 50/50 with conventional jet fuels to gain the aromatic and cycloparaffinic components for density and seal swell. Extraction of the aromatic components in a typical jet fuel sample is illustrated in FIG. 1. Hydrocarbon type analysis (ASTM D2425) shows that most aromatics in jet fuels are substituted single-ring aromatics (typically about 15 vol %), with several percent additional of substituted napthalenes/tetralins/indanes (bicyclics). The abscissa in FIG. 1 is related to the molecular weight of the aromatics. The 38° C. minimum flash point in jet fuel eliminates most aromatics smaller than C8. In FIG. 2, a blend of commercial Exxon solvents (AR 100/150/200) has been used to simulate jet fuel aromatics in combustion testing which is used for comparison in a number of tests.
Therefore, tests have been carried out to evaluate synthetic aromatics used for jet fuels, including: first, the quantity of aromatics that must be added to SPK or HRJ fuels to create a fully-synthetic drop-in jet fuel; second, the effect of the added aromatic components on the seal swell; third, the effect of the aromatics on combustion performance; and fourth, the effects of added aromatics on other properties, such as lubricity.
Density, Flash Point, Freeze Point
Typical SPK and HRJ fuels have densities (in g/ml), and specific gravities in the range of 0.75-76 (at 16 C/standard conditions). However, the permissible jet fuel range is 0.775-0.84. Density has a large impact on range, and there is little interest in the aviation community in fuels with densities lower than 0.775.
FIG. 3 shows the result of adding mesitylene (density 0.8652) to Sasol® IPK (iso-paraffinic kerosene), one of the conventional SPK's with a density of 0.762. Addition of roughly 13 vol % mesitylene yields a Sasol® IPK/mesitylene fuel blend which meets the minimum density specification. The main objective of creating a fully synthetic biofuel can also be achieved by adding the bio-mesitylene to a conventional HRJ fuel. In a preferred embodiment, adding about 20 vol % bio-mesitylene to a tallow HRJ fuel (POSF 6308) yields a fuel having properties shown in Table 4.
It can be seen that adding mesitylene (flash point 44° C.) lowers the flash point of the HRJ slightly, but the minimum is 38° C., so there are no flash point issues for JP-8/Jet A/Jet A-1. Adding solely mesitylene to an HRJ will not meet the current JP-5 specifications (60° C. minimum flash). The low freeze point of mesitylene lowers the freeze point of the HRJ fuel. The density is well above the lower limit.
TABLE 1Properties of 80 vol % tallow HRJ/20% mesitylene.6308 + 20%JP-8 req'tHRJ 6308mesityleneFlash point, C. >385552Freeze point, C.<−47−62−77Density0.775-0.840.7580.779Distillation/Boiling Range
There is a requirement for hydroprocessed SPKs in the current alternative fuel specification, ASTM D7566, for a minimum boiling range which is expressed in terms of the standard ASTM D86 boiling range limit as T90−T10>22° C. There is concern by engine manufacturing companies that very narrow boiling fuels (such as might be created by adding mesitylene to n-decane) might not have satisfactory combustor operability. Thus, adding a single-component aromatic component to a fuel (as opposed to a wide-boiling aromatics blend like FIG. 1) might not provide satisfactory properties. Therefore, in a preferred embodiment, the aromatic (such as mesitylene) was added only up to the jet fuel blend limit of 25 vol % at a maximum.
The 165° C. boiling point of the mesitylene tends to pull down the initial part of the boiling distribution. This can be seen in FIG. 4, where data for the 20% mesitylene blended into S-8 SPK is shown, along with several HRJs and blends (including three blends that have flown on commercial aircraft). As can be seen, several of the pure HRJs fall outside of JP-8 average range, which is the standard deviation around the 2006-2008 average of 5000 samples. However, it was unexpectedly discovered that blends (including 20% mesitylene in SPK) fall inside the typical JP-8 “experience base”.
Seal Swell
Mesitylene was blended into an SPK fuel (Sasol® IPK) to determine the effects on the swell of nitrile o-rings (the “problem” o-rings for leaks). As shown in FIG. 5, mesitylene blends with the Sasol® IPK swelled slightly less than blends with petroleum aromatics (shown in FIG. 2) and 1,2,4-trimethylbenzene, but the difference within typical variations seen at a given aromatic level. In other words, a 15% mesitylene blend fell within the range of seal swells seen for jet fuels of the same aromatic content. Thus, it appears that the current 8% minimum aromatic level in ASTM D7566 will be adequate to ensure seal swell with mesitylene blends as well as SPK and HRJ blends.
Viscosity
There are two main concerns with viscosity of the fuel blend. First, maintaining viscosity below low temperature limits (e.g., 8 cSt at −20° C.) is required to ensure Auxiliary Power Unit (APU) and engine cold start performance. Second, use of jet fuel in diesel engines is enabled by a viscosity above 1.3 cSt at 40° C. As shown in FIG. 6, the low viscosity of the mesitylene decreases the viscosity at low temperatures (good for aircraft) and at high temperature (bad for diesels). Thus, meeting the 1.3 cSt requirement in mesitylene blends of roughly 10-15% is apparently achievable, but it is driven by the viscosity of the primary synthetic SPK or HRJ component.
Cetane
Use of jet fuel in diesel engines (either aviation or ground) requires an understanding of the effect of the jet fuel composition on cetane number as well as viscosity. A requirement of ASTM D975 is a minimum cetane number of 40 for diesel fuel, although cetane number is not specifically called out in ASTM D7566 at this point. Since cetane is roughly inversely proportional to octane, it is to be expected that adding mesitylene, a high-octane avgas blending component, would drop the cetane number of the base fuel. As shown in FIG. 7, this is indeed the case, where the addition of 20% mesitylene to a 57 cetane HRJ lowers the measured cetane (ASTM D6890) to about 44. However, this reduction tracks well with the general trend of cetane reduction with aromatic content in jet fuels, so it does not exclude the use of mesitylene blends in diesel engines.
Lubricity
Lubrication performance of jet fuel between fuel-wetted parts is an important property. One expected issue with fully-synthetic fuels is lubricity. The standard test for this property is ASTM D5001 the Ball on Cylinder Lubricity Evaluator (BOCLE). Jet fuel lubricity is general thought to come primarily from trace polar impurities in jet fuel, so it might be expected that existing fully-synthetic fuels would have poor lubricity (as indeed they do). The major issue for addition of synthetic aromatics to fuel blends is the effect of the aromatic addition on the poor lubricity of the base fuel.
It is expected that fully-synthetic fuels used by the military will contain the mandated corrosion inhibitor/lubricity improver (CI/LI) additive. Thus, a series of tests were performed with additized mesitylene/alternative fuel blends. As shown in FIG. 8, the lubricity of 10% mesitylene blends in various additized alternative base fuels falls well within the range of experience with JP-8 and meets the JP-8 lubricity requirements (the larger the wear scar, the poorer the lubricity). Very limited testing with fuel blends without the CI/LI additive were performed, and it was typically seen that mesitylene did not significantly affect the lubricity of the base fuel. For example, camelina HRJ had a BOCLE wear scar diameter of 0.76 mm, while addition of 10% mesitylene to the HRJ reduced the wear scar to 0.75 mm.
Combustion Emissions (Specifically Soot/Particulates)
The relationship between fuel aromatic content and soot/particulate emissions is well known. Thus, it would be a surprise if the addition of mesitylene did NOT increase soot from engines (or increase the smoke point, the relative specification test). Smoke point tests were performed on mesitylene blends with Sasol IPK. As shown in FIG. 9, the addition of mesitylene to this SPK fuel did, indeed, unexpectedly reduce the smoke point (equivalent to increasing soot emissions), but in a non-linear fashion. In any case, the results were well above the 22 mm specification limit. Efforts to verify this behavior led to inconsistent results, so it was decided to compare actual engines emissions in a T63 helicopter engine. In this case, the baseline JP-8 fuel contained 16 vol % aromatics, so the emissions from a 16% blend of mesitylene in the tallow HRJ fuel were compared to this baseline JP-8.
As shown in FIG. 10, the relatively low soot emissions implied in FIG. 9 are verified in this engine test. FIG. 10 shows the reduction in particulate (soot) emission index relative to the baseline 16% aromatic JP-8. As can be seen for the camelina and tallow HRJ fuels, the soot emission index is unexpectedly, dramatically reduced. 50/50 HRJ/JP-8 blends still show roughly 50% reductions. The 16% mesitylene blend also shows significant reductions relative to the JP-8 baseline at both idle and cruise conditions, so it seems clear that addition of mesitylene to alternative fuels does not produce a sooty fuel.
Thermal Stability
SPK and HRJ fuels are extremely thermally-stable fuels, due to their extremely low contaminant content. Thermal stability was assessed in various rig tests and in the Jet Fuel Thermal Oxidation Tester (JFTOT, ASTM D3241). The jet fuel specifications require that fuel pass the JFTOT at 260 C (the higher the temperature at which a fuel passes the test, the more stable the fuel). Fuels can also be characterized by where they fail the test, or “break”—hence the highest temperature at which a fuel will pass the test is known as its “breakpoint”. A typical JP-8 breakpoint is 280° C.
The SPK and HRJ specifications require that these fuels pass the JFTOT at 325° C., at a minimum (thus the breakpoint is above 325° C.). This temperature is well above that for typical jet fuels, verifying the high thermal stability. A limited amount of thermal stability testing was performed with mesitylene, with more extensive testing performed with the aromatic blend shown in FIG. 2. Many aromatics are known to reduce fuel thermal stability although some appear to be relatively benign. In a series of tests with petroleum aromatics in various HRJ and SPK fuels, it was discovered that addition of 10, 15 and 20 vol % petroleum aromatics consistently reduced the breakpoint from >325° C. to about 280° C. for all the fuels (thus little affect of aromatic content).
Therefore, addition of petroleum aromatics above some low threshold (below 10%) reduces the thermal stability of SPK and HRJ fuels to typical jet fuel values (where the average aromatic content is 15-20%). The behavior was seen with mesitylene, where 10% mesitylene in the Syntroleum S-8 SPK fuel dropped the breakpoint down to about 280° C., or typical jet fuel levels (similar to petroleum aromatics).
The fermentation of a biomass using microbes to produce acetone and butanol was first discovered by Chaim Weizmann in 1916 and is described in U.S. Pat. No. 1,315,585 and other corresponding patents throughout the world. This process known as the Weizmann process was used by both Great Britain and the United States in World Wars I and II to produce acetone for the production of cordite used in making smokeless powder. Unfortunately, this method is energy intensive, and accordingly uneconomical.
A number of methods are known for making mesitylene from acetone and include, for example:
(1) Liquid phase condensation in the presence of strong acids, e.g. sulfuric acid and phosphoric acid as described in U.S. Pat. No. 3,267,165 (1966);
(2) Vapor phase condensation with tantalum containing catalysts as described in U.S. Pat. No. 2,917,561 (1959);
(3) Vapor phase condensation using as catalyst the phosphates of the metals of group IV of the periodic system of elements, e.g. titanium, zirconium, hafnium and tin as described in U.S. Pat. No. 3,946,079 (1976);
(4) Vapor phase reaction in the presence of molecular hydrogen and a catalyst selected from alumina containing chromia and boria as described in U.S. Pat. No. 3,201,485 (1965);
(5) Vapor phase reaction using catalysts containing molybdenum as described in U.S. Pat. No. 3,301,912 (1967) or tungsten as described in U.S. Pat. No. 2,425,096, a vapor phase reaction over a niobium supported catalyst with high selectivity. The catalyst is preferably made by impregnating a silica support with an ethanolic solution of NbCl5 or an aqueous solution of Nb in order to deposit 2% Nb by weight and by calcining the final solid at 550° C. for 18 hours. At 300° C., the condensation of acetone produces mainly mesitylene (70% selectivity) at high conversion (60-80% wt) as described in U.S. Pat. No. 5,087,781.
It is also known in the art to dimerize acetone to ultimately form isopentane. This process involves first dimerizing acetone to form diacetone alcohol which is then dehydrated to form mesityl oxide. The mesityl oxide then undergoes gas phase reformation/hydrogenation to form isopentane.
It is also known from U.S. Pat. No. 7,141,083 to produce a fuel comprising mesitylene and straight-chain alkanes (i.e., hexanes, heptanes, octanes, nonanes and the like) from plant oil, such as corn oil. The composition of corn oil is shown in Table 1 below. The predominant components of corn oil are stearic, palmitic, oleic, and linoleic acids of the free fatty acids.
It is an object of the present invention to provide biogenic fuels that effectively replace petroleum-based fuels currently used in engines.
It is another object of the present invention to provide fully renewable fuels for other internal combustion/heat engines as well.
It is a further object of the present invention to provide high energy renewable fuels for use in turbines and other heat engines by the same methodology; the energy content and physical properties of the renewable components being tailored to the type of engine to be fueled.
It is another object of the present invention to provide a binary mixture of components which meet the technical specifications for turbine engines.
It is another object of the present invention to provide a non-petroleum based aviation fuel which meets the technical specifications of ASTM International for petroleum-based turbine fuels.
It is still another object of the present invention to provide a process for the production from a biomass of the components of binary chemicals and ternary mixtures which satisfy the technical specifications for both turbine and diesel engines.